The present invention relates to gas burner apparatus and methods of burning fuel gas in the presence of air and furnace flue gas to create a stable flame while suppressing the formation of nitrous oxides (“NOx”) and carbon monoxide (“CO”).
Various types of gas burners have been developed and successfully utilized with a combination of both diffusion and pre-mix capabilities. A pre-mix approach mixes both the air and fuel gas to a homogenous mixture prior to combustion within the confines of the furnace. A diffusion approach injects the fuel gas into an air stream wherein mixing takes place without a venturi. The flame is stabilized close to the point of exit, creating both thermal and prompt nitrous oxides. Both approaches are routinely utilized to ignite and combust a given fuel gas to generate heat within a process burner.
In both pre-mix and diffusion-type burners, an optimum approach can be defined to reduce both thermal and prompt nitrous oxide formation. The emission of nitrous oxide and carbon monoxide gases by process burners as well as other combustion equipment is closely regulated by the government. The government is constantly pushing for better methodology to further reduce emissions from current combustion equipment.
In order to lower the production of nitrous oxides and other potentially polluting gases, various improved gas burner apparatus are being developed. In one approach, all of the air together with primary fuel is burned in a first zone and the remaining fuel is burned in a second zone. In this staged fuel gas approach, the staged fuel becomes dilute with furnace flue gas diluting a substantial portion of the gas stream during combustion thereby lowering the combustion temperature of the gases. The nitrogen in the air and flue gas functions as a heat sink in that it absorbs heat from the flame. The flue gas can come from the furnace stack (external flue gas) or from the furnace itself (internal flue gas). Lowering the combustion temperature of the gases lowers the formation of nitrous oxides in the produced flue gases. Examples of low NOx burners and associated methods are shown by U.S. Pat. Nos. 5,275,552 (issued to John Zink Company on Jan. 4, 1994) and 6,729,874 B2 (issued to John Zink Company on May 4, 2004), which are incorporated by reference herein.
Staged combustion and dilution of the fuel gas create additional concerns that need to be addressed, including non-combustibility and flame instability. An appreciable amount of air or flue gas is needed to dilute the flame enough to achieve a sufficient reduction in nitrous oxide formation. However, if the fuel gas is overly diluted, it may be difficult to ignite or the ignited flame may become unstable. Flame instabilities can create further instabilities capable of destabilizing the entire furnace.
Coanda surfaces have been utilized in flares wherein significant flow rates at elevated pressures are a reality. A Coanda surface is merely a curved surface designed for the adherence of a fluid. Fluid streams injected on or adjacent to a Coanda surface tend to adhere to and follow the path of the surface. The negative pressure and viscous forces pull the fluid against the surface. The fluid stream is spread into a relatively thin film or sheet, which allows proximate fluids to be mixed in with the fluid stream in a very efficient manner. The additional surface area imparted to the gas significantly enhances mixing. In a flare, for example, which may emit tens of thousands of pounds of waste gas per hour, fast mixing is desirable. As a result, Coanda surfaces and the Coanda effect are commonly used in flare apparatus as it eliminates the need for steam, blowers and related equipment.
However, Coanda surfaces have not been incorporated into low NOx process burner apparatus. Burner components are smaller and entail much lower gas flows than flare components. As a result, Coanda technology has not been actively applied to process burners. Also, many refinery operators have not changed the refinery furnaces due to the expense involved therewith. As a result, replacement burner assemblies often have to fit into existing furnace boxes which defines the performance criteria the burner must meet (for example, the length and diameter of the flame).
By the present invention, various ways have been discovered to utilize Coanda surfaces in low NOx staged fuel gas burners to greatly improve the efficiency of the burners while avoiding problems such as non-combustibility and flame instability.